About the Voltage Divider Calculator
A voltage divider uses two resistors in series to produce a smaller voltage from a larger supply, one of the most common building blocks in analog circuit design. This calculator returns the divided output voltage and the current flowing through the pair, given a supply voltage and the two resistor values.
How It Works
Enter the input voltage (Vin) along with the two series resistors R1 and R2. The output voltage is taken across R2, calculated as the fraction of Vin set by R2's share of the total resistance. The same total resistance is also used to report the current flowing through the series pair, converted into milliamps.
Formula & Methodology
The two resistors carry the same current because they're in series, so by Ohm's law that current is I = Vin/(R1+R2). Multiplying that current by R2 gives the voltage dropped across R2 alone, which is Vout. Because R1 and R2 only appear as a ratio in the Vout formula, scaling both resistors up or down together while keeping their ratio fixed changes the current and power dissipated but leaves Vout unchanged.
Examples
9V Battery Scaled to 6V
A 9V supply through R1 = 1kΩ and R2 = 2kΩ produces Vout = 9 x (2000/3000) = 6V across R2, with 3mA flowing through the series pair.
Equal-Ratio Sensor Bias
A 5V logic supply split with two 10kΩ resistors gives Vout = 2.5V, the midpoint reading often used to bias a sensor input, drawing 0.25mA of quiescent current.
Advantages
- Reports both the scaled output voltage and the resulting circuit current in one calculation, useful for checking power dissipation alongside voltage.
- Removes the need to manually rearrange Vout = Vin x R2/(R1+R2) each time resistor values change during prototyping.
- Flags an invalid resistor combination (R1 + R2 at or below zero) before a circuit gets wired incorrectly.
Common Mistakes
- Swapping R1 and R2 - Vout is measured across R2 specifically, so putting the wrong resistor in that slot inverts the intended scaling.
- Ignoring loading effects: attaching a low-impedance load, such as a multimeter on a sensitive range or a microcontroller pin with internal pull resistors, effectively changes R2 and drops Vout below the calculated value.
- Picking resistor values so small that the circuit wastes noticeable current as heat, or so large that the output becomes overly sensitive to whatever load is attached.
Edge Cases to Watch For
- The only guard is against R1 + R2 being zero or negative overall; a single negative resistor value still computes as long as the sum stays positive, which isn't physically realistic.
- This models an unloaded, open-circuit divider - it assumes nothing draws current from the Vout node, while a real load resistor connected in parallel with R2 pulls Vout below the calculated value.
- The reported current is the divider's own bleed current through R1 and R2, not any current available to supply a downstream load.
Common Use Cases
- Hobbyists scaling a higher battery voltage down to a safe microcontroller analog input range.
- Electronics students confirming a divider design meets a target output voltage before breadboarding it.
- Technicians troubleshooting a sensor circuit by checking whether a measured voltage matches the divider's expected output.